While in the geothermal reservoir underground, geothermal brine is in chemical equilibrium with most of the minerals present in the reservoir rock, and chemical constituents of these minerals are dissolved in the brine.
When steam is separated from produced geothermal brine, the temperature drops, and pH increases. The solubility of several minerals commonly precipitating from geothermal brine or depositing as scale is highly sensitive to brine pH; these include calcite (calcium carbonate) and other carbonates, magnesium silicates, calcium silicates, clay minerals, and the sulfides of zinc, iron and lead. Scale deposits must be controlled to avoid serious operational problems.
FIG. 1 illustrates relevant parts of a geothermal power generating system known in the prior art as a “steam-binary hybrid.” Hot brine is obtained from a well. Steam separated from the brine drives a steam turbine, and additional heat is extracted from the brine in a binary heat exchanger, vaporizing a nonaqueous working fluid which drives another turbine (not shown).
Commonly, a pump is installed downhole in a geothermal well to push the brine to the surface. Cavitation inside the pump causes separation of a vapor phase, increasing brine pH and creating conditions that favor rapid precipitation of calcite. Chemicals that inhibit the precipitation of calcite are injected downhole in each well below the pump intake to prevent scale deposition inside and near to the pump. Because decreasing temperature increases the solubility of calcite, and because the steam separators are not especially sensitive to scale deposition, this treatment usually suffices to control scale deposition in the steam separators as well.
However, the binary heat exchanger is highly sensitive to scale deposition and precipitation of solids. Scale deposited on the water side of the heat exchange surfaces seriously degrades heat transfer performance and increases the pressure drop of brine flowing through the binary heat exchanger, while solids precipitated from the brine tend to accumulate at the tube sheets, plugging the tubes. Magnesium silicate is a frequent constituent of scale that forms inside the heat exchange tubes; iron silicates, calcium silicates, calcium carbonate, clay minerals and sulfide minerals are also encountered.
Continuing precipitation of solids can also damage the reinjection wells by accumulating in the wellbore or cementing the cracks and pores in the reservoir rock, reducing permeability.
Scale deposition in the binary heat exchanger and reinjection wells must be controlled to maintain proper performance of the power plant. Commonly, an inhibitor specially developed to inhibit deposition of silicate minerals is added to the brine before it enters the binary heat exchanger. Silicate inhibitors are expensive, and using them dramatically increases operating costs. This problem of high treatment cost has been recognized from the time that steam-binary hybrid geothermal power plants were first operated some twenty years ago, but remains unsolved.